As shown in the photo, dahlias attract insects, some of which are harmful to the dahlia. Dahlias are also subject to several diseases which are described below.
This is a picture of Crown Gall on a stored tuber. The tuber didn’t show any physical signs until it started to shoot.
Brown spots appearing on dahlia leaves are a worrisome sight for any gardener. These unsightly blemishes indicate that your prized dahlias are under attack. But before you panic, realize that not all leaf spots are fatal. By examining the shape color, and location of the spots you can often diagnose the culprit. With that knowledge, you can take action to salvage your dahlias.
What Causes Brown Spots on Dahlia Leaves?
Brown spots have several potential causes, including:
- Fungal diseases like anthracnose, botrytis, and cercospora leaf spot
- Bacterial infections such as bacterial leaf spot (pseudomonas)
- Environmental stresses like too much sun, drought, or nutrient deficiencies
- Pest damage from insects feeding on leaves
The most common causes of brown spots on dahlia leaves are fungal diseases Different fungal pathogens create spots with distinct characteristics that aid diagnosis Bacterial leaf spot also causes brown spots, sometimes with a yellow halo. Spots from environmental factors appear random with no defined shape or pattern. Chewing insect pests make small holes or notches in leaves, which then turn brown.
How to Identify Different Types of Brown Leaf Spots
Carefully examine the shape, edges, color, and pattern of the spots to determine the likely cause:
Anthracnose
- Irregularly shaped brown spots with tan centers
- Spots enlarge and merge as disease progresses
- Most severe on lower leaves after prolonged wet weather
Botrytis Blight
- Starts as yellowing or browning on leaf edges and tips
- Spots enlarge to form big dead patches on leaves
- Gray fungal growth may be visible in wet conditions
Cercospora Leaf Spot
- Small brown spots with tan or gray centers
- Spots are circular with defined edges
- Can be mistaken for fungal shot hole
Bacterial Leaf Spot
- Small, watersoaked brown spots on leaves
- Spots may be surrounded by a yellow halo
- Blackening and death of leaf veins in advanced stages
Environmental Stress
- Random, irregular brown spots without patterns
- Usually seen first on oldest leaves
- Whole leaf may turn brown and crispy
Pest Damage
- Tiny holes chewed in leaves or notches along leaf edges
- Stems and buds may also be chewed or blemished
- Visible insect pests like earwigs, slugs, or caterpillars
How to Prevent Brown Spots on Dahlia Leaves
Preventing brown leaf spots starts with promoting general plant health. Follow these tips:
- Plant in full sun with good air circulation
- Avoid wetting foliage when watering
- Stake tall varieties to prevent wind damage
- Apply balanced fertilizer according to package directions
- Clean up fallen leaves and debris regularly
For fungal diseases, preventive fungicide applications may be warranted in wet seasons. Treat with neem oil, sulfur, or chemical fungicides. Test products for phytotoxicity on a small area first.
Control insect pests by handpicking, using insecticidal soaps, or applying Bt for caterpillars. Prevent slugs and snails with barriers and traps. Exclude earwigs with row covers.
Monitor for environmental stresses and adjust care accordingly. Increase water and shade during heat waves. Watch for nitrogen deficiencies and fertilize as needed.
How to Treat Brown Spots on Dahlia Leaves
If brown leaf spots do appear:
- Remove badly infected leaves and destroy – don’t compost
- Improve air circulation by thinning inner foliage
- Treat remaining foliage with fungicides or bactericides
- Increase watering but avoid wetting leaves
- Apply fertilizer with extra nitrogen to help recovery
For bacterial infections, treat with copper sprays or Serenade biofungicide. Remove and destroy severely infected plants to prevent spread. Sterilize tools after cutting out diseased growth.
When fungal leaf spots are caught early, you can often save the plant by removing the worst leaves. Just be diligent about monitoring and treating to keep the disease from taking hold.
When to Get Help for Brown Spots on Leaves
Seek expert input if:
- Spots rapidly worsen despite treatment
- You’re unsure of the diagnosis
- Whole plants quickly decline
The local extension office can assess samples to identify stubborn plant diseases. They may also recommend better treatment options for severe cases.
Don’t wait until the last minute if fungal or bacterial leaf spot is taking over your dahlia patch. The sooner you can correctly diagnose and treat brown spots, the better chance your plants have of surviving. Act quickly at the first signs of trouble and you can continue enjoying your dahlias all season long.
How to Identify Dahlia Viruses and Prevent Virus Infection from Destroying Your Plants
FAQ
How to get rid of brown spots on dahlia leaves?
What do overwatered dahlias look like?
How do you treat dahlia disease?
How to treat dahlia fungus?
How do you know if a dahlia has a fungus?
Watch for fungal disease during wet periods. Another common dahlia disease is Sclerotinia, also known as “white mold.” Early symptoms of this fungus include browning and/or wilting stems. If stems are sliced open laterally, a string of tiny black spores can confirm a sclerotina dianosis.
What causes dark spots on Dahlia leaves?
Dahila Smut is caused by Entyloma dahliae, which appears as pale spots on the leaves during wet conditions. Pale leaf spots with dark centers that begin on lower branches and trend upward can be symptoms of dahlia smut disease. Like most fungi, smut presents during wet periods or if irrigation is focused on leaves rather than roots.
How do you know if a dahlia has a pest?
Disinfect tools between plants. Control thrips. It can be difficult to differentiate between the ailments that affect dahlias. Dahlia pest and disease symptoms can be similar and are often interrelated. In most cases, the only reliable way to diagnose a specific disease is by sending a sample out for testing.
How do you know if a dahlia is rotting?
They also give off a distinct, foul odor. If you walk past your plants and catch a whiff of something extremely unpleasant, crouch down and look at the base of the stem. Give it a gentle squeeze, and if it feels soft, your dahlia is likely suffering from stem rot.